difftool: implement the functionality in the builtin

This patch gives life to the skeleton added in the previous patch.

The motivation for converting the difftool is that Perl scripts are not at
all native on Windows, and that `git difftool` therefore is pretty slow on
that platform, when there is no good reason for it to be slow.

In addition, Perl does not really have access to Git's internals. That
means that any script will always have to jump through unnecessary
hoops, and it will often need to perform unnecessary work (e.g. when
reading the entire config every time `git config` is called to query a
single config value).

The current version of the builtin difftool does not, however, make full
use of the internals but instead chooses to spawn a couple of Git
processes, still, to make for an easier conversion. There remains a lot
of room for improvement, left later.

Note: to play it safe, the original difftool is still called unless the
config setting difftool.useBuiltin is set to true.

The reason: this new, experimental, builtin difftool was shipped as part
of Git for Windows v2.11.0, to allow for easier large-scale testing, but
of course as an opt-in feature.

The speedup is actually more noticable on Linux than on Windows: a quick
test shows that t7800-difftool.sh runs in (2.183s/0.052s/0.108s)
(real/user/sys) in a Linux VM, down from  (6.529s/3.112s/0.644s), while on
Windows, it is (36.064s/2.730s/7.194s), down from (47.637s/2.407s/6.863s).
The culprit is most likely the overhead incurred from *still* having to
shell out to mergetool-lib.sh and difftool--helper.sh.

Still, it is an improvement.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>